All but one objects not visible, yet in library, rendering. Alt+H doesnt work
It sounds like you're in local view. You can tell by looking in the top left corner of your viewport, it says something like User Perspective (Local). As xlxs said, you can use / to switch local view on or off. But be sure to use the one on the keypad. The regular one doesn't work.

It could also be that you moved the other objects to a different layer.
Below your viewports there are two 5 by 2 grids. If there are dots in a square it means there are objects on that layer. You can either show multiple layers at the same time by shift left clicking the grid squares and making them visible (the squares turn dark), or you can left click a square to only show that layer. If you select objects in a certain layer and press m, you can move them to another layer.